As handheld communication devices are often used in non-ideal ambient acoustic environments, it is a challenge to provide maximum acoustic output under nominal or high leak conditions while not exceeding certain sound levels which may damage an ear of the user (over the short or long term exposure to such sound levels). A speaker is a transducer which is commonly provided with a device as an audio output device for such communication devices.
One currently-used technique to provide output sound levels that are in a safe range defines hard limits for sound levels generated by a speaker for a communication device set for worst case conditions when the speaker is exposed to the ear in a sealed environment, namely when the speaker is tightly acoustically sealed around the ear. This level of seal is a function of the pressure applied to the properly positioned device against the user's head. However, this leads to a compromise maximum level under normal or low seal conditions, which may lead to producing output sound signals that are at too low of a volume level, such that the user cannot hear the sound signals in high ambient noise environments.
There are standardized levels for acceptable measured acoustic shock levels for communication devices. For example, Underwriter Laboratory (UL) 60950 requirements for acoustics shock and safety in a mobile phone indicate that a mobile communication device must not generate a sound level in a headset receiver that exceeds 118 dBA (i.e. has a higher absolute value that exceeds 118 dBA) when the mobile phone is driven with a full scale square wave signal at a downlink input. Different thresholds are set for different receivers: 125 dBA for handsets, 118 dBA for headsets and 121 dBA for insert earphones.
To test output level of a speaker, the mobile communication device is placed on a Head and Torso Simulator (HATS) artificial ear with a force of 13 newtons (N), and the maximum sound pressure is measured. It will be appreciated that 13 N is a very strong force, emulating a situation where a user of a mobile device is pushing the mobile phone very firmly against his ear. This is referred to as a “low leak condition”. However, under normal conditions, a user typically will not press the mobile phone against his ear with this level of force. In the 3GPP Technical Specification 26.131 standard, a normal user force for standard loudness measurement is set to be 8 N. As such, a force of 8 N is used to press against the ear for a HATS test to measure against maximum volume standards. This is commonly referred to as a Receiver Loudness Rating (RLR)=−13 dB.
As such, mobile phone designers typically design acoustic operating characteristics relating to gain and frequency response to tune responses of the acoustic system to achieve their desired results at the 8 N force.
Typically, the maximum sound pressure achieved for 13 N position is higher than the maximum sound pressure achieved for the 8 N position because the ear coupling is stronger as the applied force increases. As a result of this difference, at a pressure of 8 N, the maximum acoustic level achieved is typically lower than what is allowed by standards.
There is a need for acoustic systems to account for changes in maximum acoustic output levels at the receiver under non-ideal conditions, such as a low-leak condition.